We are attempting to determine the influence on each amino acid residue in an approximately 60 residue Kazal type inhibitory domain upon the inhibitory activity of the domain. To this end we are determining the amino acid sequences of numerous avian ovomucoids, crocodilian ovomucoids, avian and crocodilian ovoinhibitors and of pancreatic trypsin inhibitors and seminal acrosin inhibitors. We are particularly focusing our attention upon finding domains, which differ by only one amino acid residue, but show differences in inhibitory activity. When this task proves too difficult we attempt to prepare the needed domains by semisynthetic enzymatic replacement of individual amino acid residues. When the needed domains are available we determine for them the value of the equilibrium constant for peptide bond hydrolysis, K-hyd and the association equilibrium constant, K-assoc, as well as the kinetic rate constants for association k-assoc and k-assoc and dissociation k-off and k-off. We are presently developing techniques for very rapid determination of these parameters since we are interested in at least 100 inhibitory domains and in about 10 serine proteinases (trypsin, kallikrein, acrosin, chymotrypsins A, B, C, elastases 1 and 2, subtilisins Carlsberg and BPN').